Characterizing resonant species using nuclear magnetic resonance (NMR) can include identifying different properties of a resonant species (e.g., T1 spin-lattice relaxation, T2 spin-spin relaxation, proton density). Other properties like tissue types, and super-position of attributes can also be identified using NMR signals. These properties and others may be identified simultaneously using magnetic resonance fingerprinting (MRF), which is described in Magnetic Resonance Fingerprinting, Ma D et al., Nature 2013:495, (7440):187-192.
Conventional magnetic resonance (MR) pulse sequences include repetitive similar preparation phases, waiting phases, and acquisition phases that serially produce signals from which images can be made. The preparation phase determines when a signal can be acquired and determines the properties of the acquired signal. For example, a first pulse sequence may produce a T1-weighted signal at a first echo time (TE) while a second pulse sequence may produce a T2-weighted signal at a second TE. These conventional pulse sequences typically provide qualitative results where data are acquired with various weightings or contrasts that highlight a particular parameter (e.g., T1 relaxation, T2 relaxation).
When MR images are generated, they may be viewed by a radiologist and/or surgeon who interprets the qualitative images for specific disease signatures. The radiologist may examine multiple image types (e.g., T1-weighted, T2-weighted) acquired in multiple imaging planes to make a diagnosis. The radiologist or other individual examining the qualitative images may need particular skill to be able to assess changes from session to session, from machine to machine, and from machine configuration to machine configuration. Additionally, the reviewer or interpreter may need to make their assessment in light of artifacts or other suboptimal image components caused by, for example, inhomogeneity in the main magnetic field B0.
Unlike conventional MRI, MRF employs a series of varied sequence blocks that simultaneously produce different signal evolutions in different resonant species (e.g., tissues) to which the RF is applied. The term “resonant species”, as used herein, refers to an item (e.g., water, fat, tissue, material) that can be made to resonate using NMR. By way of illustration, when RF energy is applied to a volume that has bone and muscle tissue, then both the bone and muscle tissue will produce an NMR signal. However the “bone signal” and the “muscle signal” will be different and can be distinguished using MRF. The different signals can be collected over a period of time to identify a signal evolution for the volume. Resonant species in the volume can then be characterized by comparing the signal evolution to known evolutions. Characterizing the resonant species may include identifying a material or tissue type, or may include identifying MR parameters associated with the resonant species. The “known” evolutions may be, for example, simulated evolutions or previously acquired evolutions. A large set of known evolutions may be stored in a dictionary. The ability to match evolutions may, in some instances, be somewhat compromised by varying conditions in an MR environment including, for example, an inhomogeneity in the main magnetic field B0.
Different pulse sequences have been employed with MRF. For example, both balanced steady state free precession (bSSFP) and Quick Echo Split Technique (QUEST) have been employed with MRF. QUEST is described in Heid O, Deimling M, and Huk W. QUEST—A Quick Echo Split NMR Imaging Technique, Magn Reson Med 1993; 29:280-283. MRF with both bSSFP and QUEST have demonstrated the efficiency of MRF in estimating multiple relaxation parameters simultaneously. Unfortunately, some MRF pulse sequences may have had some susceptibility to an inhomogeneity in the main magnetic field B0.